To overcome fading, wireless networks typically may employ diversity techniques, for example, channel interleavers, multiple antennas, frequency-hopping, and so on. Some systems may utilize cooperative diversity in which users partner in sharing their antennas and other resources to create a virtual array through distributed transmission and signal processing. However, in a slow fading environment, a burst of error may significantly degrade error performance and thus negatively impact the reliable decoding of the received signal. If the system can tolerate a certain delay, retransmitting the signal using Automatic repeat request (ARQ) protocols may help to enhance communication reliability. In Hybrid ARQ (HARQ) protocols, error detection and correction may be combined in order to obtain better reliability and throughput. Cooperative diversity concepts can further increase the performance of HARQ protocols. In this setting, when the initial frame decoding attempt at the receiving end fails, another transmitter, designated as the cooperating relay, performs signal retransmission over a higher-quality channel. This approach guarantees that the destination terminal receives an independently-faded version of the frame after retransmission from the cooperating relay, increasing the likelihood of reliable decoding at the receiver, and reducing the expected number of retransmissions necessary for successful decoding of the transmitted message.
Frame structures and transmission mechanisms may be implemented to enable client relay functionality, relaying executed by the mobile stations, in the next version of Worldwide Interoperability for Microwave Access (WiMax2) networks while also meeting requirements of the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard and ensuring compatibility and minimal impact to implementing the IEEE 802.16e standard. In such a setting, in the case of a downlink transmission by the base station to a particular intended subscriber station, another subscriber station, designated as the client relay, can decode the base station's transmission and transmit a reencoded version to the intended subscriber station in the uplink subframe and thereby enable the intended subscriber station to enjoy better quality of service. In such client relaying, the link between the base station and client relay station and the link between the client relay station and the intended subscriber station typically have much better channel quality than the link between base station and intended subscriber station. By enabling the intended subscriber station to receive multiple faded copies of the signal transmission from the base station, the use of client relaying leads to a better end-to-end throughput performance through cooperative diversity gains.
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